On the Sign in method tab, enable the Facebook sign-in
method and specify the App ID and App Secret you got from Facebook.

Then, make sure your OAuth redirect URI (e.g. my-app-12345.firebaseapp.com/__/auth/handler)
is listed as one of your OAuth redirect URIs in your Facebook app's settings page on the
Facebook for Developers site in the
Product Settings > Facebook Login config.

Authenticate with Firebase

Integrate Facebook Login into your app by following the
developer's documentation. When you configure the
LoginButton or LoginManager object, request the
public_profile and email permissions.
If you integrated Facebook Login using a LoginButton, your
sign-in activity has code similar to the following:

KotlinAndroid

After a user successfully signs in, in the LoginButton's
onSuccess callback method, get an access token for the
signed-in user, exchange it for a Firebase credential, and authenticate with
Firebase using the Firebase credential:

If the call to signInWithCredential succeeds, you can use the
getCurrentUser method to get the user's account data.

Next steps

After a user signs in for the first time, a new user account is created and
linked to the credentials—that is, the user name and password, phone
number, or auth provider information—the user signed in with. This new
account is stored as part of your Firebase project, and can be used to identify
a user across every app in your project, regardless of how the user signs in.